NEW CONSTRAINTS ON THE TIMING AND RATES OF FLUVIAL INCISION AND CHANNEL PROFILE FORM ALONG THE WESTERN SLOPE OF THE ROCKIES

The correspondence of high topography and low seismic velocity mantle in the Colorado Rockies compels testing the hypothesis that dynamic support of high topography may have driven tilting and differential rock uplift across the western slope of the range. Here, we explore relationships among the timing and magnitude of Late Cenozoic fluvial incision, the steepness of river longitudinal profiles, and mantle velocity domains. New ⁴⁰Ar/³⁹Ar ages on basalts capping the Tertiary Browns Park Formation range in age from ~11-6 Ma and provide paleosurfaces from which we reconstruct ~500-900 m of incision along tributaries of the Green River in northern Colorado (White, Yampa, and Little Snake rivers). The magnitude of incision generally decreases toward the north along the flank of the Rockies. Quantitative analysis of river profiles of the upper Colorado River and its tributaries (e.g. Gunnison and Dolores rivers) reveals relatively steep channels. These profiles overlie mantle of low seismic velocity and are associated with ~1000-1500 m of incision over the past ~10 Ma. In contrast, tributaries of the Green are two to three times less steep within reaches of comparable bedrock lithology; these profiles overlie mantle of progressively lower seismic wave velocities to the north. The correspondence of steep river profiles and deep incision suggests that the fluvial systems are dynamically adjusting to an external forcing and is not readily explained by a putative increase in erosivity associated with late Cenozoic climate change. Rather, fluvial incision may be best explained by relative base level fall/headwater uplift between the Colorado Rockies and the Colorado Plateau. However, alternative models involving very young integration of the Green and Colorado rivers also need to be considered.